Numerous methods and devices for detecting particles, such as soot or dust particles, are known from the related art.
It is known in the art to use two electrodes placed on a ceramic material to measure a concentration of particles, such as soot or dust particles, in an exhaust gas. This may be accomplished, for example, by measuring the electrical resistance of the ceramic material that separates the two electrodes. To be more precise, the electric current, which flows between the electrodes in response to the application of an electric voltage thereto, is measured. Electrostatic forces cause the soot particles to be deposited between the electrodes and, over time, to form electrically conductive bridges therebetween. The more of these bridges that are present, the more the measured current increases. The result is an increased short-circuiting of the electrodes.
Sensors of this kind are used, for example, in an exhaust branch of an internal combustion engine, such as in a diesel combustion engine. These sensors are usually located downstream of the exhaust valve, for example, downstream of the diesel particulate filter. In the future, heightened environmental awareness, as well as legal regulations will necessitate that the soot emissions be monitored during operation of a motor vehicle and that the functionality of the exhaust-gas aftertreatment devices, such as a particulate filter, be assured, for example. This type of monitoring of the functionality is generally referred to as on-board diagnostics. The loading level of diesel particulate filters must also be predictable. A resistive soot sensor provides one practical way of doing this. It uses the change in the resistance of an interdigital electrode structure caused by soot deposits to detect the soot. Due to the operating principle thereof, the resistive soot sensor is classified under the accumulative sensing principles.
The German Patent Application DE 10 2006 002 111 A1 describes a soot particle sensor, for example, that is constructed using a ceramic multilayer technology, i.e., from a plurality of superposed layers. The multilayer technology makes possible a compact and rugged design where the functions of soot sensing, heating and temperature sensing can be realized on different superposed planes. The interdigital electrodes of platinum are placed on the top side of the ceramic substrate using screen printing techniques. Between these, soot bridges form by the application of an electric potential difference, the sensor signal, therefore, being generated in response to a short circuit. The sensitivity of the sensor is essentially limited by the distance between the interdigital electrodes, the smallest possible distance being desired.
In spite of the numerous advantages of the known, related art methods and devices for detecting particles, they still have potential for improvement. The above described ceramic sensor design has a comparatively low thermal conductivity, for example. Accordingly, the heating element must be adapted for a higher heating power to ensure a temperature that suffices for burning off accumulated particulate from the sensor and for providing the required dynamic response. Current consumption is thereby increased. In addition, the electrical signal current on the order of μA, which can be obtained using these sensors, requires a potential difference of more than 40 V, so that the signal processing necessitates a separate control unit. Therefore, the application of the sensor proves to be complex. Manufacturing sensors of materials, such as silicon, does, in fact, allow a fine structuring in a photolithographic process of the interdigital electrode measuring region and, thus, makes it possible to do without a sensor control unit to achieve cost advantages. In terms of process technology, however, the substantially greater thermal conductivity in comparison to ceramic entails considerable outlay to thermally insulate the interdigital electrode measuring region well enough to maintain the manageability of the heating power required for the regeneration process. Moreover, silicon is too expensive to use as a material in sensor regions where the advantages thereof do not have any effect, or where other properties, such as brittleness, are rather even disadvantageous.